It is desirable to discard and replace a tonometer prism after it has contacted and become wetted with tears of a pair of eyes being examined. Discarding used prisms can prevent transfer of bacteria, viruses or prions from one patient to another. For this purpose, our previous U.S. Pat. Nos. 5,070,875, 6,179,779, and 6,471,647 and our pending application Ser. Nos. 09/811,709 and 10/178,987 have suggested a few ways of making tonometer prisms readily replaceable. The desirability of replacing applanation prisms is not limited to tonometers. As revealed in our application Ser. No. 10/178,987, applanation prisms are also useable in ophthalmologic instruments examining eyes for purposes other than tonometry.